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New Zealand shows even strong building codes are no match for monster earthquake, experts say [Update]

February 25, 2011 | 8:32
am

The devastation and loss of life in the Christchurch, New Zealand, quakes offer some sober lessons for California, earthquake experts said.

The damage shows that a sharp earthquake in a highly vulnerable area can get the better of strong seismic safety codes.

“If the dart lands right on you, it generates intense shaking and a lot of buildings that we think are safe turn out to not be safe,” said Susan Hough, seismologist with the U.S. Geological Survey.

The quake’s epicenter occurred less than six miles from the center of Christchurch, New Zealand’s second-largest city, far closer than a 7.0 quake in September, which had an epicenter about 30 miles away and resulted in no deaths.

Also, the highest ground acceleration recorded was greater than 2G, or twice the acceleration of gravity –- which would make Tuesday’s quake among the most powerful in terms of ground-shaking acceleration on record, said Hough. It was strong enough to throw objects in the air.

“There’s only a handful of records of shaking” as strong as 2G, Hough said. “That’s quite extreme shaking.… Some earthquakes just have stronger shaking that affects buildings.”

By contrast, the ground shaking in the 6.7 Northridge quake in 1994 was less, recorded at 1.7G, Hough said. Hough has estimated that the strongest ground shaking in Port-au-Prince in the Haiti earthquake in 2010 was only about 0.5G.

Among the buildings destroyed in the quake were the Pyne Gould Guinness Building, which is described by local media as one of Christchurch’s best-known office towers, the Canterbury Television building, and the spire of the century-old Christ Church Cathedral. Staffers at The Press newspaper in Christchurch were trapped as part of the 102-year-old building collapsed, the newspaper reported.

Hough said it appears that some of the buildings that collapsed are fairly modern, which, “to me, says that we still have some things to learn.” She noted that during the 1971 earthquake in Sylmar, Olive View Hospital collapsed, even though it had just been built; earthquake scientists also discovered after the 1994 earthquake in Northridge that some of the welds in steel frame buildings weren’t as strong as previously thought.

“In this case, I strongly suspect the accelerations just exceeded the design values,” Hough said. As recently as the 1980s, seismologists had believed that earthquakes couldn’t generally produce ground accelerations greater than about 0.5G. But scientists began understanding that ground accelerations could be much stronger after reviewing new data from newly constructed earthquake monitoring stations.

Hough said Tuesday’s quake offers a lesson in humility to California, showing what can happen when a quake is centered so close to the city center and populated by dense structures, some relatively new, and some historic.

Thomas Jordan, director of the Southern California Earthquake Center at USC, said Christchurch was devastated in large part because of the shallowness of the quake, and the fact that the strongest shaking occurred precisely in the location of the city’s downtown area.

“It’s really the fact that you had a lot of buildings very close to the very strongest hypocenter, which means the strongest shaking was right there in town,” Jordan said.

Another question is how very tall buildings in California would fare in a big earthquake, particularly the long period of booming and shaking, roughly similar to the low tones in music. “We don’t have a lot of observations of how modern tall buildings will perform in really big earthquakes,” Hough said.

There are other buildings in California whose vulnerabilities in quakes are well-known, such as multi-unit condos and apartment buildings with tuck-under parking. They are vulnerable because their first floor lacks a wall that would help keep the building from toppling in a quake, Hough said.

Los Angeles has enacted an ordinance that has required bracing of unreinforced masonry buildings, Hough said. But other municipalities haven’t enacted such requirements, although state law does require unreinforced masonry buildings in California to have signs posted warning about their condition.

In 2003, a quake that rocked California’s Central Coast killed two men in Paso Robles, when the roof of a 19th-century building collapsed on them. Farther north, parts of the 19th-century Mission San Miguel collapsed.

[Updated at 8:55 a.m.: In Christchurch, Jason Ingham was preparing for a seminar on earthquake building standards Tuesday afternoon New Zealand time when the hotel where he was staying began to shake. He and his colleagues from the University of Auckland had studied damaged structures after the last Christchurch quake, which struck in September of last year. But he said they could feel immediately that Tuesday’s quake was more intense.

And while most of the damage last time occurred to unreinforced masonry buildings, many modern buildings were damaged Tuesday, said Ingham, who is an associate professor of civil engineering.

“Our instinct is that this exceeded the loads that even the modern buildings were designed for. We are almost certain,” said Ingham. “The assumption is that an earthquake of this size would have caused damage in any modern city anywhere.” ]